1. Field of the Invention
This invention relates to a control method and control device for controlling an actuator to cause a head to track a disk in eccentric rotation in a disk device for reading data from, or for reading from and writing to, disk storage media by means of a head.
2. Description of the Related Art
Disk storage devices, such as magnetic disk devices and optical disk devices, are widely used as storage devices for computers and other systems. In such disk storage devices, eccentricity of the disk media occurs. This eccentricity arises because the center of rotation of the disk media on which positional information is stored is shifted from the center at the time positional information is recorded.
In a sector servo design, positional information (servo information) used to determine the position of an actuator is recorded in each sector of the tracks of the disk surface. If the rotational center of the disk coincides with the rotational center when positional information is recorded, then ideally, no eccentricity occurs.
However, in reality, the centers of rotation do not coincide, and eccentricity occurs. Possible causes of this include the use of disk media with servo information written by a different device, thermal deformation of the disk media and spindle axis, and shifts in the disk media due to shocks from outside. When such eccentricity exists, it appears to the actuator that sinusoidal disturbances at an integral multiple of the rotation frequency are added. Hence engineering is necessary to correct this eccentricity.
In servo control, control amounts are calculated from position errors. Hence in principle, servo control can be used to follow the disk eccentricity. However, when the amount of eccentricity is large, time is required to follow the eccentricity using servo control alone. Hence the amount of disk eccentricity correction is measured in advance and stored. The stored eccentricity correction amount is provided to the actuator, in addition to the control amounts of the servo control system, as a feed-forward current, to correct for eccentricity.
For example, methods of magnetic head position control using an eccentricity estimation observer have been disclosed in detail in Japanese Patent Laid-open No. 7-50075 (U.S. Pat. No. 5,404,235) and in Japanese Patent Application 10-185046.
The observer comprises an eccentricity estimation observer, comprising an actuator model and an eccentricity model; the subsequent state is predicted from the error between the detected position and the estimated position, from the control current, and from state variables, and moreover a control current is created from the predicted state. In this way, eccentricity is corrected for in realtime, so that prompt compensation for eccentricity is possible. In such eccentricity correction methods, it has been proposed that different eccentricity correction parameters be used for each disk surface; a single eccentricity correction parameter is stored for each disk surface, and this is read, and eccentricity correction amounts are calculated according to the state. That is, a common eccentricity correction parameter is used for a single magnetic disk surface, and the eccentricity correction amount for each track of the magnetic disk surface is calculated.
However, using the prior art, there are a number of problems.
First, when using a rotary actuator, the trajectory described by the head is arc-shaped, and does not coincide with the disk radial direction. Hence the influence of the disk deviation is different when the head is at inner circumferences on the disk and when it is at outer circumferences. Consequently, the optimal eccentricity correction amount differs for inner and outer circumferences on the disk.
Conventionally, in order to compensate a common eccentricity correction amount for such different eccentricity correction amounts, the zero-cross point of the open-loop characteristics of the feedback system is raised, for rapid convergence on the optimum eccentricity correction amount at the track position. However, in sampling servo control the sampling servo frequency must be raised in order to raise the zero-cross point. In order to raise the sampling servo frequency, the number of servo signals per track must be increased, and so there is the problem that the storage capacity of one track is reduced.
Second, when a disk written with position signals by an external servo track writer is stalled into a disk device, and in disk devices the disk media of which can be exchanged, the distance from the center of the actuator of the device used to write position information to the disk to the center of the head read/write core, and these distances in the device using the disk, are different. That is, track widths are 1 xcexcm or less, and it is difficult to improve the precision of device assembly to correspond to this. Consequently the number of tracks for eccentricity correction is increased, and time is required to follow disk eccentricity when the amount of eccentricity correction is common among disk surfaces, and so there is the problem that seek times cannot be shortened.
An object of this invention is to provide a head position control method, control device for disk device and disk device to obtain the optimum eccentricity correction amount at each track on a disk surface.
Another object of this invention is to provide a head position control method, control device for disk device and disk device to optimize the amount of eccentricity correction at each track on a disk surface, without increasing the sampling frequency.
Still another object of this invention is to provide a head position control method, control device for disk device and disk device to obtain the optimum eccentricity correction amount at each track on a disk surface, even when the eccentricity is large.
In order to perform this object, the head position control method for disk device of this invention comprises a step of reading sector position information of the disk with a head; a step of detecting the current position of the head by demodulating the position information; and, a step of calculating, a control amount corresponding to the position error between the target position and the current position, and the eccentricity correction amount for the disk, and calculating an actuator control amount for a rotary actuator for moving the head from the control amount and the eccentricity correction amount. In the above-mentioned calculation step, the eccentricity correction amount is calculated by reading the eccentricity correction information corresponding to the head position from stored eccentricity correction information for each of a plurality of regions divided in the radial direction on the disk, corresponding to the head position.
The head position control device of this invention has a head for at least reading information on a disk storage media; a rotary actuator for moving the head; a control circuit for calculating a control amount of the rotary actuator, based on position signals read from the disk storage media by the head; and memory for storing eccentricity correction information for each of a plurality of regions divided in the radial direction of the disk. The control circuit reads eccentricity correction information corresponding to the head position from the memory, and calculates the eccentricity correction amount.
In this invention, when a rotary actuator is used, there is a large difference between eccentricity amounts at the inner and outer circumferences of the disk, and so by dividing the disk into a plurality of regions, and setting eccentricity correction information for each region, optimal eccentricity correction can be performed according to the head position. Consequently optimal eccentricity correction becomes possible for each track, without raising the sampling frequency. By this means it is possible to have a sector servo correct for eccentricity without reducing the storage capacity.
The head position control method further comprises a step for saving the eccentricity correction amount for the current position at the start of seek operation, and a step for reading the eccentricity correction information for the region of the target position of the seek operation. And the calculation step comprises a step for calculation of the eccentricity correction amount according to the eccentricity correction information which has been read. Before the start of a seek operation, the eccentricity correction information is changed, so that there is no need to change the eccentricity correction information during seek control. Consequently there is no increase in the operations performed in seek control, and so optimal eccentricity correction is possible while maintaining fast seek control.
In the head position control method for disk device, the saving step comprises a step of converting the eccentricity correction amount for the current position into eccentricity correction information for the reference sector of the track of the current position, and a step of storing the converted eccentricity correction information. And the readout step has a step of converting the eccentricity correction information which has been read into eccentricity correction information for the sector of the target position. Because eccentricity correction information is stored for the reference sector, eccentricity correction information is easily calculated for each sector, and only a small amount of eccentricity correction information need be stored.
In the head position control method for disk devices, the calculation step comprises a step of calculating the control amount using an observer to estimate the position, velocity, and eccentricity correction amount of the head, in accordance with the position error. Because an observer is used, high-speed following of the eccentric track is possible even when using small amounts of eccentricity correction information.